Did you take the same file, after loading it on your 2007 machine, back to the 2002 machine? If so, this is most likely the reason for failure to save.
Once you go up, you can't go back down with relation to Office documents. They release versions far apart with large application changes that alter the format of the documents. Older files will open with newer software, but once opened, they usually cannot go back. Of course, this is only under save conditions. Simply opening a file and reading it will not alter the format from the older version. As long as you don't save it, no worries.
If this is not the case, my recommendation is to open the file, make the changes and save it under another file name. This will cause the system to re-create the file and possibly correct any corruption that may be occurring.